We have obtained from a rabbit-mouse hybridoma a cDNA clone whose sequence agrees with the available amino acid sequence of the kappa light chain secreted by the hybridoma. Using variable and constant region fragments of the cDNA clone as probes several clones hybridizing to each probe have been obtained from a rabbit sperm genomic library. These clones are currently being characterized. The activation of specific genes in B cell development are being studied using DNA methylation analysis and DNAase sensitivity. In collaboration with Dr. Stanley Korsmeyer of NCI, several leukemia/lymphoma lines representing B-lineage cells at different stages of differentation are under analysis using probes for kappa, epsilon and Mu immunoglobulin genes. In order to study a non-immunoglobulin gene whose activity may be developmentally regulated we are attempting to clone the human gene for the J chain which links IgM monomers. A human clone which cross-hybridizes with the mouse J chain gene is currently being sequenced.